Aluminum anodizing method



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Hn keks, K/ch Passau Kik United States Patent O 3,418,222 ALUMINUMANODIZING METHOD Fred C. Schaedel, Compton, Calif., assignor to Murdock,Inc., Compton, Calif., a corporation of California Filed Feb. 28, 1966,Ser. No. 530,508 6 Claims. (Cl. 204-58) This invention relates toelectrolytic anodizing of aluminum, including aluminum alloys, and inparticular, to a new and improved anodizing process and apparatusespecially suited for producing `a hard anodize finish.

A variety of problems are encountered in the production of anodizedcoatings on aluminum parts, including burning of the parts, excessivetime and voltage requirements, and relatively soft finishes. It is anobject of the present invention to provide a new and improved processand apparatus which substantially reduces or eliminates such problems.

It is a particular object of the invention to provide a new and improvedprocess and apparatus for producing a hard coating while operating forshorter periods of time,

at lower electrolyte temperatures and concentrations and at highercurrent densities. A further object is to provide such a process andapparatus which will utilize conventional electrolytes either with orwithout additives.

In the conventional anodizing process, the part to be anodized is placedin a tank of electrolyte and an electric current is passed through thepart and electrolyte. The D.C. power supply is connected across the partand tank. The initial voltage typically may be volts DC. providing acurrent density at the part in the order of to 50 amperes per squarefoot. The applied voltage will be periodically increased until thedesired coating thickness is obtained. Typically the voltage increasemay be 1 volt per minute except in the critical range where the increasemay be l volt per two minutes or l volt per four minutes. The criticalrange is the time at which burning of the part is most likely to occur.This is the point in the process when the part is nearly completelycovered with the initial oxide layer leaving relatively small unoxidizedlayers having a relatively low surface resistance and hence beingsusceptible to a localized high current density which may produceburning of the part. The voltage range in which the critical area occursfor any particular material, electrolyte composition, concentration,tank design and running procedure is readily determined by testing. Itis the practice in anodizing processes to make the voltage increases atlonger intervals during this portion of the process and also to closelywatch the current and voltage indicating meters for sharp variations incurrent or voltage. Such varations indicate incipient or actual burningand the applied voltage may be reduced to protect the parts. It is aspecific object of the invention to provide a new and irnproved processand apparatus which substantially eliminates the problems of burningduring the critical period and hence substantially eliminates the needfor manual supervision of the current and voltage indicators.

It is an object of the invention to provide an anodizing process whereinthe part to be anodized is placed in an electrolyte and an electriccurrent is passed through the part and electrolyte in pulses, with theamplitude of the current pulses being varied at a frequency about 1%()the pulse rate. A further object is to provide such a process whereinthe current pulses are half wave rectified alternating current pulses,preferably obtained from half wave rectification of the commercial powersource. An additional object is to provide such a process wherein thecurrent is applied at a pulse rate in the range of about to about 125pulses per second and the current amplitude is varied at a frequency ofabout to about 150 cycles per minute to produce a current change ofabout 5 to about 17 percent.

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It is an object of the invention to provide a new and improved apparatusfor operating an anodizing tank and including a half wave rectifiercoupled between an A.C. power source and the tank, and circuit means forcyclically varying the amplitude of the rectifier current in the tank. Aparticular object is to provide such an apparatus incorporating acurrent limiting resistance, and switching means for cyclicallyconnecting the resistance in series between the rectifier output and thetank, and bypassing the rectitier output around the resistance.

'Other objects, advantages, features and results will more fully appearin the course of the following description. The drawing merely shows andthe description merely describes a preferred embodiment of the presentinvention which is given by way of illustration or example.

In the drawing:

FIG. 1 is an electrical schematic of a preferred form of the apparatusof the invention; and

FIG. 2 is a diagram illustrating the current wave forms produced by theapparatus of FIG. l.

The present invention -utilizes conventional electrolytes land followsthe conventional practice of periodically increasing the voltage duringthe anodizing operation. An additive may be used with the electrolyte ifdesired. In addition, in the process of the invention, the current isapplied in pulses, typically half Wave rectified alternating currentpulses produced by half wave rectification of the commercial powersource. Also, the amplitude of the current pulses is varied at afrequency about 1&0 the pulse rate. It has been -found that anodizingwith pulse current and with period variation in amplitude of the currentproduces a harder coating while permitting operation at lower voltagesand for shorter periods of time.

An apparatus suitable for performing the process is illustrated inFIG. 1. A part 10 to be anodized is suspended in a tank of electrolyte11. The power source may be the commercial volts 60 cycles per secondpower which is connected at terminals 12, 13. Of course, other powersources can be utilized. The input terminals 12, 13 are connectedthrough an isolating transformer 14 to an autotransformer 15. Oneterminal of the autotransformer 15 is connected directly to the tank.The moving arm of the autotransformer is connected to a diode unit 16which functions as a half wave rectifier. The rectifier output isconnected to the part 10 through a contact set 17 and an ammeter 18. Avariable resistance 19 is connected in parallel with the contact set 17.A voltmeter 20 is connected across the power supply to provide a measureof the voltage at the tank.

Means is provided for cyclically opening and closing the contact set 17.Typically this may comprise a cornmutator circuit 25 which cyclicallyengergizes and deenergizes `a low voltage relay coil 26. Contact set 27of the low voltage relay controls application of power to coil 28 of ahigh voltage relay which in turn controls the contact set 17.

When the commutator 25 closes the circuit between the 6 volt D C. supplyat terminal 30 and ground, coil 26 is energized, contact set 27 isclosed, coil 28 is energized, and contact set 17 is closed and therectifier output is directly connected to the part being anodized. Whenthe commutator opens the circuit between terminal 30 and ground, contactset 17 is opened, switching the resistance 19 in series with the partand electrolyte. It is Arecognized that a wide variety of switchingdevices may be utilized in lieu of the particular circuitry illustrated.The specific embodiment of FIG. 1 provides for commutating at relativelylow voltage and current while providing for current control atrelatively high voltage and current.

The operation of the apparatus of FIG. 1 is illustrated 3 in FIG. 2. Thesolid curve 35 illustrates the half wave rectified current pulsesapplied to the part and electrolyte with the Contact set 17 closed. Thedashed curve 36 illustrates the current pulses with the contact set 17open and with the resistance 19 connected in series.

The pulse rate for the current pulses should be in the range of about 45to about 125 pulses per second. As a matter of economy and convenience,commercial A C. power ordinarily is utilized in the anodizing processand the pulse rate will correspond to the frequency of the power source,which in most instances is 60 cycles per second providing a pulse rateof 60 pulses per second.

The current amplitude is varied at a frequency about 1/60 the pulse rateand preferably in the range of about 50 to about 150 cycles per minute.The most preferred range utilizing a 60 pulse per second rate is at afrequency of about 70 to about 90 cycles per minute.

In anodizing equipment, the magnitude of the current is a function ofthe size of the tank, the size of the parts being treated and the numberof parts, and hence the actual magnitude of current will vary over awide range for different installations. Therefore figures on currentamplitude are significant only when related to a speciiic installation.However, the current density at the surface of the part being treated,ordinarily measured in amperes per square foot, does provide forcomparison between different installations. At the start of an anodizingrun, the current increases from an initial value to a higher operatingvalue. The current remains at this higher yoperating value for the restof the run, although it may vary somewhat during the progress of therun. In the process of the invention, the change in current asillustrated in FIG. 2, should be in the range of about 5 to about 17percent of the operating value and preferably is in the order of 8 to 14percent, with the most desirable range being about 10 to 12 percent.That is to say, the difference between I1 and I2 should be about 11percent Of Il.

The following are specific examples of the process of the invention.

EXAMPLE 1 Test panels 4 x 4 x 1A inches of 2024-T3 aluminum alloy wereanodized using a conventional 10 percent sulfuric acid electrolyte withan additive and operated in the range of 25 to 30 F. The additive was:disodium EDTA, 0.1% of electrolyte by weight; sodium lauryl sulfate, 2Occ. per 40 gallons of electrolyte; and Benax surfactant, 100 cc. per 40gallons of electrolyte. A half wave rectified 60 cycles per secondvoltage source was used. The voltage was initially set at volts. Thecurrent initially was about 10 amperes and increased to an operatingValue of about amperes. The current amplitude was cyclically reduced andraised 2 amperes at a frequency of 80 cycles per minute. The voltage wasperiodically increased as set out in the following chart, producing thecoating thicknesses as indicated in the chart.

Raise voltage from- .At rate of Coating thickness,

inches 20-24 1 volt/min 24A 1 volt/4 min. 26-30-. 0. 001 30-40- 0. 002-55 0. 003

Parts of 6061-T6 aluminum alloy were anodized using the sameelectrolyte, pulse rate, current change frequency 4 and amplitude, andinitial starting voltage as in Example 1. The voltage was periodicallyincreased as set out in the following chart to produce the coatingthicknesses indicated.

At rate ofinches EXAMPLE 3 Parts of 7075-T6 aluminum alloy were anodizedin the same manner as set `out in Examples 1 and 2, with the voltagebeing periodically increased as set out in the following chart toproduce the thicknesses indicated.

Coating thickness Raise voltage rom- At rate ofinches 1 volt/min 1volt/2 min TABLE I Haterial New process Conventional 2024-T3 .002-30 min.00245-50 min. .003-45 min .003-75-80rnin. 6061-'16 .002-20 min .002-40nun. 7075-T .002-15-20 min .002-35 min.

Table II provides a comparison of the voltages required to produce a0.002 inch thick coating on three different materials.

TABLE II Material New process Conventional 20H-T3 38-40 volts 50-60volts.

do Do. 075-T6 35 volts o0 volts The results as set out in Tables I andII were achieved using the new process as set out in Examples 1, 2 and 3and, for the conventional process, using the same tank and meters withidentical parts and with a direct current power supply and a 15 percentsulfuric acid electrolyte at 30 to 35 F. The current density duringanodizing in the new process was in the order of to 100 amperes persquare foot. The current density in the conventional process was in theorder of 35 to 50 amperes per square foot. The coatings produced by theconventional process exhibited a loss of .001 to .0015 inch in a 40,000cycle Taber test. The coatings produced by the new process eX- hibited a.0007 to .0008 inch loss in a 40,000 cycle Taber test. No burningproblem was encountered during the new process and the process wasoperated at the high current density. The usual critical control areawas encountered in the conventional process, requiring close manualsupervision of voltage and current and a limitation on the maximumcurrent density.

The test results show that the process of the invention will produce aharder coating in a shorter period of time and at lower voltages and atlower concentrations of electrolyte than the conventional processes.

Although exemplary embodiments of the invention have been disclosed anddiscussed, it will be understood that other applications of theinvention are possible and that the embodiments disclosed may -besubjected to various changes, modifications and substitutions withoutnecessarily departing from the spirit of the invention.

I claim as my invention:

1. In a process for anodizing aluminum wherein the tric current ispassed through the part and electrolyte, the v steps of:

applying the current in pulses; and

varying the amplitude of the current pulses at a frequency about 1&0 thepulse rate.

2. A process as defined in claim 1 wherein the current is applied at apulse rate in the range of about 45 to about 125 pulses per second andthe amplitude is varied at a frequency of about 50 to about 150 cyclesper minute to produce a current change of about 5 to about 17 percent.

3. A process as dened in claim 2 wherein the current pulses are halt`wave rectified alternating current pulses.

4. A process as dened in claim 1 wherein the current pulses are halfwave rectied alternating current pulses.

5. A process as defined in claim 1 wherein the current pulses are halfwave rectified alternating current pulses at a power source frequencyand the amplitude is varied at a frequency of about 70 to about 90cycles per minute to produce a current change of about 5 to about 17percent.

6. 'A process as defined in claim 1 wherein the variation in currentamplitude is produced by cyclically switching a resistance in serieswith the part and electrolyte.

References Cited UNITED STATES PATENTS 1,388,874 8/1921 Mershon 204-582,443,599 6/ 1948 Chester 42041-52 2,901,412 8/1959 Mostovych et al.204-211 2,920,018 1/ 1960 Miller 204-58 XR 2,930,741 3/ 1960 Burger etal 204-228 XR 2,951,025 8/1960 Mostovych et al. 204-211 3,020,219 2/1962 Franklin et al. 204-58 FOREIGN PATENTS 446,112 1/ 1948 Canada.

654,299 12/1962 Canada.

821,896 11/1951 Germany.

JOHN H. MACK, Primary Examiner. G. KAPLAN, Assistant Exalmz'ner.

U.S. Cl. X.R. 204-228 UNITED STATES PATENT OFFICE CERTIFICATE 0ECORRECTION Patent No. 3,418,222 December 24, 1968 Fred C. Schaedel dthat error appears in the above identified It is certifie atent arehereby corrected as patent and that said Letters P shown below:

table, line l thereof,

Colurn 3, in the heading to the Raise voltage from to- "Raise voltagefTom-' should read Signed and sealed this 17th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Ir.

Commissioner of Patents Attesting Officer WILLIAM E. SCHUYLER, JR.

1. IN A PROCESS FOR ANODIZING ALUMINUM WHEREIN THE PART TO BE ANODIZEDIS PLACED IN AN ELECTROLYTE AND AN ELECTRIC CURRENT IS PASSED THROUGHTHE PART AND ELECTROLYTE, THE STEP OF: APPLYING THE CURRENT IN PULSES;AND VARYING THE AMPLITUDE OF THE CURRENT PULSES AT A FREQUENCY ABOUT1/60 PULSE RATE.